There are cases where a precatalyst having oxidizing ability is provided in the exhaust passage of an internal combustion engine upstream of an exhaust gas purification apparatus such as a NOx storage reduction catalyst (which will be hereinafter referred to as a NOx catalyst), a particulate filter (which will be hereinafter referred to as a filter) or the like. In such cases, when the temperature of the exhaust gas purification apparatus is to be raised in order to recover the capability of the exhaust gas purification apparatus, a reducing agent is supplied to the precatalyst. The reducing agent thus supplied is oxidized by the precatalyst, and the temperature of the exhaust gas purification apparatus is raised by heat of oxidation generated thereby.
In an arrangement disclosed in Japanese Patent Application Laid-Open No. 2001-342879, a NOx catalyst is provided in the exhaust passage of an internal combustion engine, and a precatalyst is provided in the exhaust passage upstream of the NOx catalyst. This patent document 1 discloses a technology of controlling the air-fuel ratio of the exhaust gas in accordance with the degree of deterioration of the precatalyst when NOx stored in the NOx catalyst is reduced. Furthermore, Japanese Patent Application Laid-Open No. 2001-073749 discloses a technology in which when the temperature of a catalyst provided in the exhaust passage of an internal combustion engine is raised upon starting the vehicle, a time period over which the temperature of the catalyst is raised is determined in accordance with the degree of deterioration of the catalyst.